Mount St. Helens began to show signs of volcanic unrest in March of 1980. ...unrest began on March 15, 1980. From the 15th through the 21st over 100 earthquakes were recorded. It was a magnitude 4.1 earthquake recorded on March 20 that provided scientists with the first early warning sign that Mount St. Helens might be preparing for an eruption for the first time since 1857.
The most current activity at Mount St. Helens started on September 23, 2004 heralded by a swarm of small earthquakes.